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Astronomy 101 study guide 2 - Astronomy 101 review Chapter...

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Astronomy 101 review Chapter 1 Cosmic Address Since we cannot feel the constant motion of the Earth as it rotates, it led past astronomers to believe that the Sun, moon, and planets revolved around us. (Geocentric) Our place in the universe is called our cosmic address. Earth is a planet in our solar system, which consists of the Sun and all objects that orbit it which include the planets and their moons and many other small objects such as asteroids and comets. The sun is a star; The sun and stars make up a very small part of a huge, disk- shaped collection of stars called the Milky Way galaxy. A galaxy is a great island of stars in space. Our solar system is located more than halfway from the galactic center to the edge of the galactic disk. Our Milky Way is one of the two largest galaxies among 40 others in the Local Group. Groups of galaxies with more than a few dozen members are often called Galaxy Clusters. Regions in which galaxies and galaxy clusters are most tightly packed are called Superclusters (clusters of galaxy clusters.) All of these structures create our universe , the sum total of all matter and energy, encompassing clusters and everything within them. Order of Cosmic Address [ Universe>Local Supercluster>Local Group>Milky Way Galaxy>Solar System>Earth] Define: Astronomical Unit: The average distance between Earth and the sun, which is about 150 kilometers. More technically, 1 AU is the length of the semi- major axis of Earth’s orbit. Observable universe: The portion of the entire that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe. Light-year : The distance that light can travel in 1 year, which is about 9.46 trillion kilometers. Solar Day : Our 24-Hour day, is based on the time it takes the Sun to make one circuit around the local sky. Sidereal Day : About 23 hours and 56 minutes, is the time of an entire day relative to stars (how long it takes for a star to go from it’s highest point in the sky one day to its highest point the next day) Sidereal Year : The time it takes for Earth to complete one orbit relative to the stars. Tropical Year: Since our calendar is based on the cycle of the seasons, which we measure as the time from the spring equinox one year to the spring equinox the next year. About 20 minutes shorter than the sidereal year.
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Noon : The precise moment when the sun is highest in the sky (on the meridian) and the sundial casts its shortest shadow. (This would be basing time on the sun’s actual position in the local sky, as is the case when we use a sundial [measurement of apparent solar time . ]) Orbital speed- how fast earth travels around the sun.
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